Nail courses – Joints
Joint is a connection which allows two extremities of bones to make contact or move.
- Fibrous joints
– joint surfaces are irregular (knobbly, irregularly flat, etc.),
– have short accessory ligaments and capsules so they permit little mobility,
– like the rib-sternum joint, the rib-vertebrae joint, the joints of hip girdle, tarsal and carpus,
- Synovial joints
– have regular joint surfaces,
– permit relatively large movement.
According to the number of connected bones we distinguish:
- simple and
- compound joints.
Synovial joints can be classified according to the number of the axes’ movements:
1. Uniaxial joints
These are also called hinge joints. These are e.g. interphalangeal joints, elbow and ankle. They permit only flexion or tension.
2. Biaxial joints
These can be:
- so-called ellipsoidal joints e.g. the wrist,
- so-called saddle joints e.g. thumbs, carpal and metacarpal joints.
These permit flexion-extension, adduction – abduction.
3. Multiaxial or free joints
These can be:
- arthrodialis joints or
- ball-and socket joints.
Such joints are shoulder and hip joints. These permit movement in every direction.
Connection of bones can be: interrupted (joints) and continuous or uninterrupted junctions (those of connective tissue, cartilage, ligament).
Components of joints:
1. Bone ends:
Parts of bones, the joints of which make contact, can have different shapes, however, one end is most often convex (the so-called joint head) and the other one connected to it is concave (the so-called acetabulum). If the bone ends are flat or irregular then we do not refer to them as a joint head or acetabulum, but simply bone ends. The connected bone ends generally unite with surfaces that are the moulding of one another.
2. Articular cartilage:
Covers the articular surfaces that are served for connecting the bone ends to each other. The hyaline cartilage layer can be thicker (1-2 mm) or thinner (some tenth mm) depending on the size and weight imposed on the joint. Its surface facing the joint is generally smooth, however, where the form of articular surfaces do not match, the difference is equalized by the fibrocartilaginous disc (sternum-clavicle joint, manducatory joint, knee joint).
3. Articular capsule:
It is a sack-like connective tissue which seals the two joint bone ends hermetically from its surroundings. It has loose reserve folds within joints that make large movements in order to permit them to move. Joints of less mobility have a tight articular capsule. It is made up of two layers, an outer fibrous layer and an inner layer. This inner layer (the lining of the joint) has a velvety, smooth surface where the so-called synovia is produced, a liquid which fills the capsule ensuring the oiling of the joint and the nourishing of articular cartilage.
4. Articular cavity:
The articular capsule completes the gaps between the cartilaginous articular surfaces with a sealed joint cavity. Actually, the articular cavity is not a cavity as the cartilaginous surface and the surrounding capsule are fit together completely so, in fact, there is as thin gap between them as hair. Under pathological circumstances e.g. arthritis, articular sprain, etc. the cavity can be filled with liquid. It contains some drops of greasy (unction) liquid
5. Articular ligaments:
The thickened bundle of the articular capsule’s fibrous layer is a separated bunch containing connective tissue.
Joint connection factors:
The closely fitting cartilage surfaces of joints cannot be separated, even by the influence of strong mechanical effects (pulling). Their connection is caused by:
1. Articular ligaments:
They have the most important role in connecting, the main ligament of hip joint would tear under about 500 kg.
2. Joint capsule.
3. Effect of air pressure:
There is a vacuum in the articular cavity so an air column of 1kg weight presses on each square metre of the articular capsule’s area which puts pressure on bone ends. Forces that keep the bone ends together can be taken over by pulling in smaller joints. In such a case, due to the vacuum, the articular capsule is folded in between the drawn apart bone ends by making a little cracking sound. Such happens when e.g. someone, out of a bad habit or because of nervousness, makes their joints crack by pulling their fingers.
4. Surrounding muscle-tones:
It is decisive that joints are surrounded by strong muscles and have loose capsules as well as accessory ligaments (e.g. in shoulder joints), (so when having muscle paralysis occurs the bone ends can go away from each other, the joint becomes „baggy”).
5.Connection of cartilage surfaces to each other.